1. Field of the Invention
The present invention relates to an apparatus for displaying information of a radar, sonar, etc. on a circular screen of a cathode-ray tube (CRT) by utilization of a polar coordinate deflection system (a PPI system) and particularly relates to a sweep termination signal generator for generating a sweep termination signal which commands the termination of a sweep signal for the deflection of an electron beam.
2. Description of the Prior Art
A conventional marine radar has its transmission position, i.e. the position of a ship per se, deviated from the central point O of a plan position indicator (PPI) display provided with a circular CRT to a backward point P to widen the range of forward supervision, as illustrated in FIG. 1. That is to say, the marine radar has an off-center function.
When display is effected by a PPI display having the aforementioned off-center function, it is desirable to terminate sweep of an electron beam by cutting unnecessary part of the sweep signal as soon as the deflection position of the electron beam reaches a position of the outermost circumferential edge of a CRT screen from a viewpoint of power consumption.
The relationship between the sweep signal and the sweep termination signal will be described hereinafter with reference to waveform diagrams in FIG. 2. When the bearing angle of an antenna is of .theta..sub.A in FIG. 1, a sweep termination signal E.sub.A is generated at the time that an electron beam deflected by a sweep signal of sweep S.sub.A effected upon occurrence of a trigger has reached a point A on the outer circumference of the PPI display, i.e. at the lapsed time of T.sub.A from the occurrence of the trigger to thereby terminate the sweep signal of the sweep S.sub.A. Similarly, when the bearing angle of the antenna is of .theta..sub.N in FIG. 1, a sweep termination signal E.sub.N is generated at the lapsed time of T.sub.N from the occurrence of the trigger. In other words, when the electron beam has reached a point N on the outer circumference of the PPI display, the sweep S.sub.N is terminated.
However, the sweep velocity during real time in a radar etc. is considerably high. Since the time required when a radio wave reciprocates one fourth of a naulical mile is 3 .mu.s (3.times.10.sup.-6 sec.), for example, a deflection velocity for deflecting an electron beam to an extent of about 20 cm during the aforementioned time comes to about 66 KM/sec. Therefore, it has been difficult to materialize an apparatus capable of rapidly accurately calculating the time that an electron beam reaches the outer circumference of the PPI display, which time varies with variation in the bearing angle of an antenna relative to an optional off-center point in the PPI display, without incurring any excess cost.
For these reasons, a conventional apparatus has laid stress on an economical point and a point of high velocity. To be specific, the conventional apparatus sets a regular square having a side of 2R with a PPI display of a radius of R having a circular CRT used in order to determine the termination of sweep and allows a sweep termination signal to be generated by means of this regular square, as shown in FIG. 3.
FIG. 3 will be mathematically explained. Assuming that the central point O of the PPI display is regarded as the origin of the rectangular coordinate axes X and Y, the absolute values .vertline.X.vertline. and .vertline.Y.vertline. of a deflection position (X, Y) kaleidoscopically varied by sweep signals from an off-center point P are calculated and, when at least one of the calculated absolute values .vertline.X.vertline. and .vertline.Y.vertline. exceeds the radius R of the PPI display, i.e. .vertline.X.vertline.&gt;R and/or .vertline.Y.vertline.&gt;R, a sweep termination signal is generated to terminate the sweep.
However, the conventional method for the termination of sweep, as illustrated in FIG. 3, exhibits low accuracy concerning the sweep termination. In the sweep when the bearing angle of an antenna is set to be .theta..sub.A, for example, the sweep is terminated at a point B in spite of the fact that the point at which the sweep is to be terminated is a point A. In this case, therefore, the sweep has been effected for a longer period of time by the distance AB than the period of time to be actually required. That is to say, the conventional method is disadvantageous in that the portion of the sweep represented by the oblique lines in FIG. 3 corresponds to excess power consumption and excess sweeping time.